The present invention relates to an electrostatic chuck for holding substrates in a process chamber.
Electrostatic chucks are used to hold substrates in various applications, including for example, holding a silicon wafer in a process chamber during semiconductor fabrication. A typical electrostatic chuck comprises an electrode covered by an insulator or dielectric layer. When the electrode of the chuck is electrically biased with respect to the substrate by a voltage, an attractive electrostatic force is generated that holds the substrate to the chuck. In monopolar electrode chucks, an electrically charged plasma above the substrate induces electrostatic charge in the substrate that electrostatically holds the substrate to the chuck. A bipolar electrode chuck comprises bipolar electrodes that are electrically biased relative to one another to provide the electrostatic attractive force.
With reference to FIGS. 1a and 1b, the electrostatic attractive force generated by electrostatic chucks 10a, 10b can be of different types. As schematically illustrated in FIG. 1a, a dielectric layer 11 with a high electrical resistance results in the generation of coulombic electrostatic forces from the accumulation of electrostatic charge in the substrate 12 and in the electrode 13 of the chuck 10a. The coulombic electrostatic force is described by the equation: ##EQU1## where .epsilon..sub.o and .epsilon..sub.r are the dielectric constant of vacuum and relative dielectric constant of the dielectric layer 11, respectively, V is the voltage applied to the electrode 13, and t is the thickness of the dielectric layer. The electrostatic force increases with increased relative dielectric constant .epsilon..sub.r of the dielectric layer 11.
With reference to FIG. 1b, Johnsen-Rahbek electrostatic attraction forces occur when an interface 14 of a low resistance dielectric layer 15 and the substrate 12 comprises an interfacial contact resistance much greater than the resistance of the dielectric layer 15, i.e., when the resistance of the dielectric layer 15 from about 10.sup.11 to about 10.sup.14 .OMEGA.cm. In these chucks, free electrostatic charge drifts through the dielectric layer 15 under the influence of the electric field and accumulates at the interface of the dielectric layer 15 and the substrate 12, as schematically illustrated in FIG. 1b. The charge accumulated at the interface generates a potential drop represented by the equation: ##EQU2## where .delta. denotes the contact resistance of the air gap 14 between the substrate 12 and the low resistance dielectric layer 15. The Johnsen-Rahbek electrostatic attractive force is much larger for an applied voltage than that provided by coulombic forces because (i) polarization in the dielectric layer 15, and (ii) free charges at the interface 14 (which have a small separation distance from the accumulated charges in the substrate) combine to enhance electrostatic force. A strong electrostatic force securely clamps the substrate 12 onto the chuck and improves thermal transfer rates. Also, it is desirable to operate the chuck using lower voltages to reduce charge-up damage to active devices on the substrate 12.
It is known to use ceramic layers to fabricate the low conductivity Johnsen-Rahbek electrostatic chucks. For example, various formulations of Al.sub.2 O.sub.3 doped with low levels of 1-3 wt% TiO.sub.2 to form low resistance ceramic layers are disclosed in Watanabe et al., "Relationship between Electrical Resistivity and Electrostatic Force of Alumina Electrostatic Chuck," Jon. J. Appl. Phys., Vol. 32, Part 1, No. 2, 1993; and "Resistivity and Microstructure of Alumina Ceramics Added with TiO.sub.2 Fired in Reducing Atmosphere," J. of the Am. Cer. Soc. of Japan Intl, Edition, Vol. 101, No. 10, pp. 1107-1114 (July 1993). In another example, U.S. Pat. No. 4,480,284 discloses a chuck having a ceramic dielectric layer made by flame spraying Al.sub.2 O.sub.3, TiO.sub.2, or BaTiO.sub.3 over an electrode and impregnating the pores of the ceramic layer with a polymer. Whereas pure Al.sub.2 O.sub.3 ceramic has a resistivity on the order of 1.times.10.sup.14 ohm cm, the alumina/(1-3 wt % titania) ceramic formulations typically have lower resistivities on the order of 1.times.10.sup.11 to 1.times.10.sup.13, and consequently are more suitable for fabricating Johnsen-Rahbek electrostatic chucks. However, one problem with such ceramic layers is that the volume resistivity of the ceramic decreases to low values with increasing temperature, which results in large current leakages that exceed the capacity of the chuck power supply.
Another problem with low resistance ceramic formulations is their low charge accumulation and dissipation response time, i.e., the speed at which electrostatic charge accumulates or dissipates in the chuck. The charge accumulation time is the time to reach electrostatic charge saturation and depends on the resistivity of the dielectric layer. Typical resistivities of conventional ceramics of greater than 1.times.10.sup.12 .OMEGA.cm result in relatively slow charging times, often as high as 5 to 10 seconds. The high resistance also results in a slow dechucking time, which is the time it takes for the electrostatic charge accumulated in the chuck to dissipate after the voltage applied to the electrode is turned off. It is desirable for the chuck to provide rapid chucking and dechucking to provide high process throughput.
Yet another problem with conventional electrostatic chucks occurs during their use in semiconductor processes that use plasma environments and, in particular, high density plasma environments. A plasma is an electrically conductive gaseous medium formed by inductively or capacitively coupling RF energy into the process chamber. High density plasmas which are generated using a combined inductive and capacitive coupling source typically comprise a thin plasma sheath having a large number per unit volume of energetic plasma ions. The high density plasma species permeate into the interfacial gap between the substrate and the chuck, or the potential differences at the backside of the substrate cause formation of glow discharges and electrical arcing at the backside of the substrate. It is desirable to have an interfacial region that is more resistant to plasma permeation and that can reduce plasma formation even when charged plasma species penetrate into the gap.
The formation of glow discharges and arcing at the interfacial gap below the substrate causes additional problems when the substrate is cooled or heated by a heat transfer gas, such as helium, supplied to the interface between the chuck and the substrate via channels in the body of the chuck. The heat transfer gas serves to enhance thermal heat transfer rates. However, the pressure of the heat transfer gas below the substrate counteracts and reduces the electrostatic clamping force exerted on the substrate. Because the semiconductor plasma processing is typically carried out at low pressures, the helium gas pressure increases the size of the interfacial gap below the substrate, causing increased permeation and penetration of the high density plasma into the gap. Additional problems occur when the heat transfer gas passes through gas holes in the chuck that are surrounded by the electrode of the chuck which is supplied by a high power AC voltage. Instantaneous changes in potentials can ionize the heat transfer gas adjacent to the gas distribution holes, particularly when the diameter of the gas hole is relatively large and provides a long mean free path which allows avalanche breakdown of gas molecules in the gas holes. Ceramic chucks typically have large diameter gas holes because it is difficult to drill small holes having diameters less than 0.5 mm because the ceramic at the edges of the holes shatters or chips off during drilling. Arcing and glow discharges within these large diameter gas holes in ceramic chucks cause deterioration of the regions adjacent to the gas distribution holes, including the adjacent dielectric layer and overlying substrate.
Thus, there is a need for an electrostatic chuck that reduces plasma glow discharges and arcing in the interfacial gap between a substrate and chuck, particularly when heat transfer gas is provided to the interface. There is also a need for an electrostatic chuck that deactivates or prevents plasma formation at the gas feeding apertures in the chuck. There is a further need for a chuck having a low conductivity dielectric covering or enclosing the electrode which provides higher electrostatic clamping forces, rapid chucking and dechucking response times, and controlled leakage of current from the electrode.